Planar display devices of various types, employing display elements such as liquid crystal elements etc, have now reached the stage of practical application, and the main requirement at present is to implement high density models of such display devices. Problems which arise with regard to the drive characteristics of such display devices can be overcome by incorporating control elements to control the application of drive signal potentials to the display elements. These control elements can be active elements such as transistors, or active elements such as diodes or varistors, with one or more of the control elements being coupled to each display element. Such a method has been discussed for example by B. J. Lechner et al in a report published in the Proceedings of the IEEE, volume 59, No. 11, pages 1566 to 1579 (designated hereinafter as reference document (1). This describes both the use of 3-terminal elements (i.e. transistors) and two-terminal elements (i.e. non-linear resistance elements) as control elements. There have been proposals to use ceramic varistor to form such non-linear resistance elements, in a report by D. E. Castleberry in the IEEE ED- 26, 1979, pages 1123 to 1128. (reference document (2). In addition, it has been proposed to use MIM type diodes for such non-linear resistance elements, for example as described by D. R. Baraff et al, in the IEEE ED-28, 1981, pages 736 to 739. (reference document 3). However, several problems have arisen with these prior art methods, which have prevented them from reaching the stage of practical application. These basically are as follows:
1. Lack of uniformity of element characteristics.
2. Large amount of stray deviations in the threshold voltage of the elements.
3. A sufficiently high level of ON current I.sub.on cannot be produced.
4. The level of OFF current I.sub.off is excessively high
5. The drive power level required is high.
6. The characteristics are not symmetrical with respect to positive and negative voltages
7. Crosstalk is high.
In the above, the ON current Ion is the level of current which must be supplied by a non-linear resistance element to a display element in response to a specific drive signal potential in order to set that display element in the activated state or non-activated state in a selected period (generally speaking, a light state or otherwise visible state), or maintain a display element in the activated state. The OFF selected Ioff is the leakage current which will flow through the non-linear resistance element from a display element in response to the absence of a drive signal potential intended to hold that display element in the non-activated state (generally speaking, a dark or otherwise non-visible state). The higher the value of Ion attainable, the higher will be the maximum frequency with which display elements can be switched between the activated and non-activated states, since this current determines the amount of charge which can be transferred to or from a display element within a specific drive time interval.
In general, such non-linear resistance elements must have a symmetrical bidirectional conductance, e.g. of the general form of that of a varistor, in order to enable display elements such as liquid crystal display elements to be driven, which require application of alternating drive signal potentials. In the following, the term "non-linear resistance element" will be applied to a device having such a conductance characteristic. The term "rectifier element" will be applied to a device having a unidirectional, i.e. a diode type of conductance characteristic.